The subject invention generally concerns the field of jitter measurement, and in particular concerns an apparatus and method for measuring jitter in a signal under test with a real time digital storage oscilloscope.
Jitter is a well-known term of art used to define the deviation from an ideal timing of an event in a signal. Jitter results in the mispositioning of the significant edges in a sequence of data bits from their ideal positions. In modern serial data communications systems, the serial data clock is not usually transmitted with the data, so jitter could cause data errors at the receiving end. It is therefore extremely important to determine the amount and kind of jitter that may be present in a signal under test. In this regard, it is noted that jitter comprises two distinct types, deterministic jitter (DJ) and random jitter (RJ). Random jitter (RJ) is unbounded in amplitude and assumed to be Gaussian. In contrast, deterministic jitter is not random and is bounded in amplitude, and comprises Intersymbol Interference (ISI), Duty Cycle Distortion (DCD), and Periodic Jitter (PJ). Note that Intersymbol Interference (ISI) is also known in the art as Data Dependent Jitter (DDJ).
ISI is data-dependent deterministic jitter caused by the time differences required for a signal to arrive at a receiver threshold when starting from different places in bit sequences (i.e., symbols). DCD is the difference in the mean pulse width of a logic xe2x80x9c1xe2x80x9d pulse compared to the mean pulse width of a logic xe2x80x9c0xe2x80x9d pulse in a clock-like bit sequence. PJ is characterized by periodic variations in edge transition times at frequencies that are uncorrelated with the data rate. The signed difference between the measured time location of each sequential data symbol transition and the nominal symbol transition time is called the Time Interval Error (TIE).
RJ and DJ accumulate differently in a serial data communication link. If the parameters that characterize each of the two categories of jitter are available, then bit error rate (BER) can be estimated. It is therefore desirable to be able to measure each of the two categories of jitter. Unfortunately, one cannot simply choose to measure one of the two categories of jitter because both categories are always mixed together and appear as xe2x80x9ctotal jitterxe2x80x9d. One must separate the two categories prior to estimating their parameters.
U.S. Pat. No. 6,356,850 B1(Wistrup, et al.) discloses a jitter separation and parameter estimation apparatus and method which is based on a Time Interval Analyzer (TIA) using an arming system. Unfortunately, a TIA with an arming system does not lend itself to incorporation in an oscilloscope without that arming system. Thus, existing oscilloscopes are capable of showing only total jitter. What is needed is a solution to the problem of measuring RJ and DJ with an oscilloscope.
A jitter separation apparatus and method, based on spectrum analysis, separates deterministic jitter and random jitter using their spectral properties. Deterministic jitter exhibits a spectrum of impulses, whereas random jitter exhibits a broad, flat spectrum. A time domain histogram and a frequency domain histogram of the signal are investigated to obtain jitter components. Bit error rate estimation is performed based on the jitter separation result.